The present invention is with respect to a process and an apparatus for compacting foundry mold-making materials, and more specially for compacting wet foundry sand, by the impacting effect of a gas pressure wave on the molding material heaped onto a pattern.
A number of different forms of process on these lines have been put forward in the past and this sort of process is in fact presently becoming increasingly economically important. Examples of the process are to be seen in the German Auslegeschrift specification 1,961,234 and the German Offenlegungsschrift specification 2,949,340. The main way in which such processes are different from each other is with respect to the way of producing the pressure wave. In the one case a highly compressed gas is kept in an inlet space or antechamber and suddenly decompressed (see the said specification 1,961,234) or in the other case the gas pressure is produced right over the mold space by the explosion of a flammable gas mixture. In addition to these two families of processes there are a number of different further forms, which are however of little interest here inasfar as they have little in common with the present invention. In all the forms of the said process the pressure wave produced makes its way towards the surface of the mold material and is then responsible for a compacting effect on the said material.
Although workers in the field do not so far have any complete picture of the cause and effect of this form of compacting operation, it is known that the compaction is caused by fluidizing effects on the one hand and the impact pressure on the other, that takes effect at the particle level in the mold making material. Because of the fluidizing effect there is a decrease in the surface forces between one particle and the next of mold material and between the particles and the pattern or the flask and the filling frame so that the forces of momentum produced on the separate particles of mold making material by the effect of the gas or the particles next thereto are responsible for a more compact packing together of the mold making material.
A further observation made has been that the degree of compaction is dependent not only of the parameters (pressure and speed) of the gas pressure wave but furthermore on the level or height of the heaped molding material. There is in fact a lower limit for the cover of molding material, under which no degree of compaction with the desired effects may be produced. On the other hand there is an upper limit for the covering of mold material, above which there is not enough compaction, more specially in the case of the back of the mold. The operation is for this reason mostly undertaken with an excess of mold making material, that in the first-noted case is responsible for an unnecessarily high mold, while in the second case there will be an unnecessary extra part of molding material that has to be taken off after compaction.